1. Field of the Invention
The present invention relates to an art of a micromachine or micro-electro-mechanical systems (MEMS) using an actuator. For example, it relates to a semiconductor integrated circuit including a switch, a variable capacitor element or the like which uses an electrostatic actuator, and a driving method of the electrostatic actuator.
2. Description of the Related Art
The structure of a MEMS switch using an electrostatic actuator has been disclosed in, for example, the specification of U.S. Pat. No. 5,578,976. In order to close the MEMS switch, a potential difference is applied across a top electrode and a bottom electrode of the electrostatic actuator so that electrostatic attraction between these electrodes surpasses the spring force of a beam to which the top electrode is fixedly attached. Normally, it is necessary to apply a potential difference of 20 V or more across the top electrode and the bottom electrode to close the MEMS switch. Hereinafter, the absolute value of this potential difference is called a voltage Vs.
In the closed MEMS switch, the top electrode and the bottom electrode of the electrostatic actuator are in contact with each other via an insulating film. In this case, since there is a potential difference of 20 V or more between the top electrode and the bottom electrode, a charge is injected into the insulating film by an FN tunnel or a Pool-Frenkel mechanism, and trapped by the insulating film. This phenomenon is called the dielectric charging of the electrostatic actuator.
When the amount of the charge accumulated in the insulating film by the dielectric charging is sufficiently large, the top electrode is attracted to the charge in the insulating film even if the potential difference between the top electrode and the bottom electrode is brought to 0 V, so that the switch can not be changed from the closed state to an open state. This phenomenon is called the stiction due to the dielectric charging.
A method of biasing the voltage between the top electrode and the bottom electrode to suppress the stiction is described in G. M. Rebeiz, “RF MEMS Theory, Design, and Technology”, Wiley-Interscience, 2003, pp. 190-191. This biasing method has the following three points.
(1) A hold voltage Vh is set lower than the voltage Vs.
(2) The polarity of the drive voltage applied to the top electrode and the bottom electrode is inverted every time (bipolar actuation).
(3) Positive and negative pulses having an amplitude Vh are continuously applied in the held state of the switch.
Here, the hold voltage Vh is a potential difference between the top electrode and the bottom electrode necessary to maintain a closed state (held state) of the switch after this switch is changed from an open state to the closed state. Since the electrostatic attraction between the top electrode and the bottom electrode is proportional to the square of the reciprocal of a distance between these electrodes, the hold voltage Vh can be lower than the voltage Vs.
The amount of the charge trapped in the insulating film can be reduced but can not be totally brought to zero by the biasing method including the above-mentioned three points. The reason is that there is a difference in the amount of the injected charge between the time in which the positive pulse is applied and the time in which the negative pulse is applied. This is attributed to the asymmetry of the charge injection mechanism. Therefore, if the switching between the pulse of the positive voltage and the pulse of the negative voltage is repeated for a sufficiently long time in the held state, the amount of the charge in the insulating film gradually increases, and the stiction finally occurs.